World Trade Organization Director General Roberto Azevedo said: “A trade war is in no one’s interests.”

But the rhetoric ramped up as Mr Trump tweeted, “Trade wars are good.”

Are trade wars good?

By Theo Leggett, business correspondent

If trade wars really were good and easy to win, the World Trade Organisation probably wouldn’t exist.

Most countries believe that negotiations are best carried out and disputes settled through a rules-based system. Introducing trade barriers on a tit-for-tat basis has the potential to harm companies on both sides.

But that’s unlikely to bother Mr Trump. His campaign rhetoric drew heavily on the perceived threat to traditional US industries from foreign interlopers acting unfairly. He’s simply continuing in that vein.

And it’s unlikely to register much with the steelworkers of Pennsylvania and Indiana. Concerned about their jobs and the future, many will welcome Mr Trump’s comments.

How does the Trump administration respond?

Mr Trump tweeted on Friday morning that the US was “losing billions of dollars on trade” and would find a trade war “easy to win”.

When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win. Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy!

Critics argue that the tariffs would fail to protect American jobs and ultimately raise prices for consumers.

But US Commerce Secretary Wilbur Ross used a can of Campbell’s Soup to defend the tariffs on Friday morning as “no big deal”.

He told CNBC the duties would have a negligible effect on the price of a tin, amounting to less than a cent.

“Who in the world is going to be too bothered?” he asked.

Who else is unhappy?

Canada, Mexico, China, Japan and Brazil also say they are considering retaliatory steps.

The prime minister of Canada – which exports more steel to the US than any other country – slammed the tariffs as “absolutely unacceptable”.

Justin Trudeau told reporters in Ontario he was “confident we’re going to continue to be able to defend Canadian industry”.

Media playback is unsupported on your device

Media captionPascal Lamy, ex-head of the World Trade Organisation, says US consumers will pay if tariffs go ahead

A Chinese foreign ministry spokesman said: “If all countries followed the example of the United States, [it] will undoubtedly result in a serious impact on the international trade order.”

Kosei Shindo, chairman of the Japan Iron and Steel Federation, said that the move would “create a negative chain reaction affecting not only steel but also other products considered to have national security implications”.

Many US companies also expressed alarm, including beer brewers, which use aluminium for canned beverages.

Anheuser-Busch InBev NV, the world’s largest beer maker, warned that Mr Trump’s plan was “going to put jobs at risk and would be against the US consumer”.